The new girl ordered sandwiches containing less fat (7)
I believe the answer is:
lighter
'containing less fat' is the definition.
(lighter in fat)
'the new girl ordered sandwiches' is the wordplay.
'new' indicates anagramming the letters.
'ordered' indicates an anagram.
'sandwiches' means one lot of letters goes inside another (goes on either side like a sandwich).
'girl' anagrammed gives 'ligr'.
'the' is an anagram of 'hte'.
'hte' placed inside 'ligr' is 'LIGHTER'.
(Other definitions for lighter that I've seen before include "The girl (anag.)" , "Smoking accessory" , "Cargo barge; smoker's accessory" , "Less weighty" , "Boat for heavy loads and fire igniter" .)